Max Planck Institute for Meteorology

Concerns that human activity was contributing to climate change and mankind's fragmentary knowledge of climate dynamics led to the foundation of the Max Planck Institute for Meteorology in Hamburg in 1975. Since then, scientists at the Institute have been studying how physical, chemical and biological processes and human behaviour contribute to global and regional climate changes. The scientists develop numerical models and measurement methods to explain the natural variability of the atmosphere, the oceans and the biosphere, and to assess the influence of land use changes, industrial development, urbanisation and other human influences. Together with the Max Planck Institute for Biogeochemistry in Jena and the Max Planck Institute for Chemistry in Mainz, they strive to provide a better understanding of the chemical and biological factors that determine the concentrations of greenhouse and other trace gases in the atmosphere, and how they interact with the terrestrial and marine biospheres.

Public debates on global warming focus on one main cause: CO2 emissions from the combustion of fossil fuels. But humankind is also changing the climate by clearing forests and through farming, forestry and animal husbandry. Together with her Research Group at the Max Planck Institute for Meteorology in Hamburg, Julia Pongratz is investigating the consequences of these activities for the climate – and how these interventions could be used to counter global climate change.

Nowhere does climate change make its presence felt more strongly than in the Arctic. The volume of sea ice there has fallen drastically in recent decades. Climate models have been far from accurate in conveying the full extent of this loss. This is set to change now – not least because Dirk Notz and his research group at the Max Planck Institute for Meteorology in Hamburg are constantly improving their understanding of the processes that influence the formation and melting of sea ice.

What will the Earth’s climate be like 10 or 15 years from now? Researchers have yet to find a satisfactory answer to this question – especially as random changes that occur in such medium-term periods play a significant role. Natural fluctuations are probably also the reason why global temperatures have hardly risen at all in the past 15 years. Jochem Marotzke from the Max Planck Institute for Meteorology in Hamburg and his colleagues all across Germany are working intensively on a system designed to generate reliable forecasts for the coming years.

Global warming is changing the world – environmentally, economically and politically. Climate service providers seek to help decision makers respond appropriately to thismultifaceted change. Our authors were significantly involved in setting up the Climate Service Center in Hamburg. Here they describe the work these kinds of institutions do and the challenges they face when it comes to communicating their information.

White caps above and below – it goes without saying that these are part of our image of the blue planet. But for how much longer? In the case of the North Pole, at least, whose cover consists entirely of sea ice, it is an essential question. After all, nowhere in the world is climate change as visible as it is in the Arctic. Never before, since reliable records have been available, was the September minimum – the expansion of the Arctic Sea ice at the end of the summer – as low as it was in 2012. The Arctic ice is not only an indicator of climate change, but also an important factor in the climate system: the smaller the ice areas become in the Arctic summer, the less sunlight is reflected and the more is absorbed by the ice-free ocean. In winter, the ice insulates the relatively warm water from the much colder air; without this “cap,” the ocean would release gigantic volumes of heat into the atmosphere. The ice cover is therefore extremely important for the temperatures at the North Pole. Dirk Notz from the Max Planck Institute for Meteorology in Hamburg would like to explain the role of the sea ice, its complex internal structure, and thus also the conditions necessary for its formation and stability. To this end, he and his team measure, among other things, the thickness of the ice on the ice floes and its composition of pockets of freshwater ice, brine and gas. All of the data is included in complex numerical simulations. The most important discovery to date: Contrary to what was originally feared, there doesn’t appear to be any tipping point in the climate system, after which it would be impossible to prevent the complete loss of the Arctic ice cap. According to the model calculations, the state of the sea ice is closely related to the prevailing climate conditions at all times. This also means that if greenhouse gas emissions continue to increase at the current rate, then by the end of the century, the Arctic will be completely free of ice in September at the latest.

From 1998 to 2012, the Earth’s surface warmed more slowly than expected. Many climate scientists explained that this slowdown was caused by the oceans, which drew heat downward, away from the surface. The authors of a new study question this view: variations in the energy radiated from the surface to space could also have caused the slowdown. Furthermore, the amount of energy required to cause a slowdown is smaller than previously thought. It is in fact considerably smaller than the observational uncertainty, which suggests that the true origin of the recent slowdown may never be discovered.

Reforestation is a widely discussed measure to combat the increase of atmospheric CO2 concentrations. Earlier studies inferred from the effects of past land use changes to those of future reforestation. The global model simulations presented here show however that the potential of reforestation to sequester CO2 in a warm, CO2-rich world may be larger than anticipated. Adaptation to climate change continues to be necessary, but climate extremes may be dampened by reforestation.

The newly developed ICON atmosphere model features two important improvements compared to older models. Firstly the choice of the non-hydrostatic equations allows to simulate small scale circulations as they occur in convective clouds and their environment. Secondly the numerical methods have been chosen such that even massively parallel supercomputers can be exploited. Both features together provide new perspectives for the research on the dynamics of convective clouds and their interaction with large-scale circulations and the global climate.

Observations suggest a hiatus in global surface temperature since 1998, whereas most climate models simulate continued warming. What causes this difference? Do climate models respond too sensitively to the increase in greenhouse-gas concentrations such as that of CO2, and thus overestimate climate change systematically? Or has the discrepancy arisen by chance? A study just published by the Max Planck Institute for Meteorology (MPI-M) gives a clear answer: There is no evidence for systematic model error.

Earth System models do not account for the limited availability of essential plant nutrients. Currently, about one third of the fossil fuel CO2 emissions is taken up by land ecosystems, but there is no guarantee that this ecosystem service will continue into the future. This study accounts for the first time for the availability of nitrogen and phosphorus on land. It shows that the omission of nutrient limitation in the models leads to an overestimation of carbon dioxide uptake by vegetation in future. Reduced CO2 uptake due to nutrient limitation will accelerate global warming.